Gene therapy for blindness

December 13, 2018 0 By FM

A four-month old male child was brought to Lilawati Hospital in Mumbai with complaints of blindness and frequent rubbing of eyes. The child had an uneventful birth history and was active, with no other clinical symptoms other than roving, or searching, movements of the eyes. His family history indicated that four out of his father’s six first cousins had congenital blindness. Brain MRI was normal, as was the fundoscopy done by an ophthalmologist. Electroretinogram (ERG), which measures the activity in the retina, showed a reduction in photoreception function in the entire retina including both rods and cones. There was also a poor response to cone stimulation as seen by visual evoked potential (VEP). Keeping the family history and clinical observations in mind, the working diagnosis was that of Leber’s congenital amaurosis (LCA).
LCA is a rare autosomal recessive genetic eye disease that appears at birth or during the first few months thereafter, and can affect males and females with an equal probability. While it affects 1-3 newborns in 1,000,000, it is one of the most common causes of blindness in children. This eye disease affects the retina that detects both light and colour, resulting in severe visual impairment early in the infancy. A common sign associated with LCA is the Franceschetti’s oculo-digital sign, characterized by the poking, rubbing, and/or pressing of the eyes.
LCA can be caused due to mutations in the genes necessary for normal vision. There are over 26 known genes that are implicated in LCA, and the list is still growing. LCA genes encode proteins important in a wide variety of retinal functions; for example, CRB1 and CRX are important for photoreceptor morphogenesis, GUCY2D and AIPL1 for phototransduction, LRAT, RDH12 and RPE65 for vitamin A cycling, etc. The mutations in some of the genes such as CEP290, CRB1, GUCY2D, or RPE65 are the most commonly associated with LCA, though 20-30% of the children with LCA have no identifiable cause.
Treatment for LCA is still primarily at the clinical trials stage, with only one FDA approved treatment for the RPE65 mutation so far. The current gene therapy involves delivering the RPE65 gene via sub-retinal injection, which allows for normal RPE65 protein to be expressed and the consequent restoration of the visual cycle. Voretigene neparvovec (Luxturna) is a novel gene therapy developed by Spark Therapeutics and Children’s Hospital of Philadelphia, USA, and is the first in vivo gene therapy approved by the FDA. Voretigene consists of the Adeno-associated virus serotype 2 (AAV2) vector containing the RPE65 cDNA. The AAV2 vector does not cause any disease and is an attractive viral vector for gene therapy. Vortigene treatment comes with its own hurdles; the cost of treatment for one eye is estimated to be a whopping $425,000! However, this is a one-time therapy, where the treatment is expected to have long-term benfits.
While Voretigene has passed clinical trials and is now an FDA-approved therapy, several other genetic therapy options for other gene mutations for LCA are currently being researched in mouse models and cell lines. GUC2YD cDNA constructs in AAV vector, when sub-retinally injected in the eyes of Gucy2e-/-Gucy2f-/- knockout (GCdKO) mice, have been shown to evoke scoptopic and photopic ERG responses. In a slightly different approach, researchers are also attempting to use Antisense Oligonucleotide-Based Splicing Correction to allow normal protein expression in case of CEP290 gene mutations. Results have been promising so far for human cell lines and in CEP290 mutant mouse models. However, these studies still need to undergo clinical trials and obtain human use approvals, and are currently far from reaching the patient’s bedside.
To confirm the diagnosis of LCA and to identify the genetic mutation in the four-month old baby, paediatric neurologist Dr. K. N. Shah sent the child’s blood sample for genetic testing using the LCA panel. This panel included 20 potential genes in which a mutation is known to cause blindness. The results indicated a mutation in the GUCY2D gene. Since, currently there is no therapy for the GUCT2D mutations, no treatment is possible for this baby. However, there is a large potential for the evolution of a treatment for the condition in the future, and it is likely that therapeutic options will start becoming available going forward.